The present invention relates to organic light-emitting devices (OLED) which produce white light.
An OLED device includes a substrate, an anode, a hole-transporting layer made of an organic compound, an organic luminescent layer with suitable dopants, an organic electron transport layer, and a cathode. OLED devices are attractive because of their low driving voltage, high luminance, wide-angle viewing and capability for full-color flat emission displays. Tang et al. described this multilayer OLED device in their U.S. Pat. Nos. 4,769,292 and 4,885,211.
Efficient white light producing OLED devices are considered low cost alternative for several applications such as paper-thin light source backlights in LCD displays, automotive dome lights, and office lighting. White light producing OLED devices should be bright, efficient, and generally have Commission International d""Eclairage (CIE) chromaticity coordinates of about (0.33, 0.33). In any event, in accordance with this disclosure, white light is that light which is perceived by a user as having a white color.
The following patents and publications disclose the preparation of organic OLED devices capable of emitting white light, comprising a hole transport layer and an organic luminescent layer, and interposed between a pair of electrodes.
White light producing OLED devices have been reported before by J. Shi (U.S. Pat. No. 5,683,823) wherein the luminescent layer includes a red and blue light-emitting materials uniformly dispersed in a host emitting material. This device has good electroluminescent characteristics, but the concentration of the red and blue dopants are very small such as 0.12% and 0.25% of the host material. These concentrations are difficult to control during large-scale manufacturing.
Sato et al. in JP 07,142,169 discloses an OLED device capable of emitting white light made by sticking a blue light-emitting layer next to the hole transporting layer and followed by a green light-emitting layer having a region containing a red fluorescent layer.
Kido et al., in Science, Vol. 267, p. 1332 (1995) and in APL Vol. 64, p. 815 (1994), report a white light producing OLED device. In this device three emitter layers with different carrier transport properties, each emitting blue, green or red light, are used to generate white light.
Littman et al. in U.S. Pat. No. 5,405,709 disclose another white emitting device which is capable of emitting white light in response to hole-electron recombination and comprises a fluorescent in a visible light range from bluish green to red.
Recently, Deshpande et al, in Applied Physics Letters, vol. 75, p. 888 (1999) published white OLED device using red, blue and green luminescent layers separated by a hole blocking layer.
However, these OLED devices require a very small amount of dopant concentrations, making the process difficult to control for large-scale manufacturing. Also emission color varies due to small changes in the dopant concentration.
It is an object of the present invention to produce an effective white light-emitting organic device.
It is another object of this invention to provide an efficient and stable white light producing OLED device with simple structure and which can be reproduced in manufacturing environment.
It has been found quite unexpectedly that white light producing OLED devices with high luminance efficiency and operational stability can be obtained by doping yellow dopant in the NPB hole transport layer and blue dopant in the ADN host emission layer.
It has also been found that by doping rubrene in the Alq electron transport layer and blue doping in the ADN host emission layer; white light can be produced.
The object is achieved by an organic light-emitting diode (OLED) device which produces substantially white light, comprising:
a) a substrate;
b) an anode disposed over the substrate;
c) a hole-injecting layer disposed over the anode;
d) a hole transport layer disposed over the hole-injecting layer;
e) a light-emitting layer doped with a blue light-emitting compound, disposed directly on the hole transport layer;
f) an electron transport layer disposed over the blue light-emitting layer;
g) a cathode disposed over the electron transport layer; and
h) the hole transport layer, electron transport layer, or the electron transport layer and the hole transport layer being selectively doped in a region which corresponds to an entire layer or a partial portion of a layer in contact with the blue light-emitting layer, the selective doping being with a compound which emits light in the yellow region of the spectrum.
This object is further achieved by an organic light-emitting diode device which produce substantially white light, comprising:
a) a substrate;
b) an anode disposed over the substrate;
c) a hole transport layer doped with a rubrene compound for emitting light in the yellow region of the spectrum;
d) a light-emitting layer doped with a blue light-emitting compound, disposed directly on the hole transport layer;
e) an electron transport layer doped with a rubrene compound for emitting light in the yellow region of the spectrum and disposed directly over the blue light-emitting layer; and
f) a cathode disposed over the electron transport layer.
The following are features and advantages of the present invention:
a simplified OLED device for producing white light by having a yellow dopant in the hole transport layer, or the electron transport layer, or both; and
an OLED device which is easy to control blue and yellow dopant concentrations since they are quite high (xcx9c2% TBP and xcx9c2% rubrene).
OLED devices made in accordance with the present invention can be produced with high reproducibility and consistently provide high light efficiency (5.3 cd/A @20 mA/cm2). These devices have high operational stability and also require low drive voltage.